Approaches and Applications of Quantitative LC-MS for Proteomics and Activitomics - LC-MS/MS in Proteomics: Methods and Applications - page 8

N -methyltransferase, which catalyses the generation of sarcosine
from glycine. The activity of this enzyme was found to be impor-
tant for cell invasion and metastasis, thus representing a new drug
target for the treatment of prostate cancer ( 25 ) . Importantly,
since sarcosine levels can be monitored in urine, it provides a
convenient biomarker for the prognosis of the disease and to per-
sonalize the treatment of prostate cancer patients based on the
metabolic status of tumours.
As mentioned above, different cancers may obtain their
energy via different metabolic pathways. Signalling pathways,
known to be deregulated in cancer, are also involved in the
homeostasis of normal cells by mediating the effects of insulin,
growth factors and other hormones that regulate energy lev-
els in cells. These pathways, which include the PI3K/Akt and
Ras/MEK/Erk pathways, affect the expression and/or activity of
metabolic enzymes. Since different cancers deregulated different
signalling pathways, the metabolic pathways by which cancer cells
obtain their energy may also be different in different tumours.
LC-MS and GC-MS-based metabolic studies support this
hypothesis. For example, certain cancer cells show an elevated
glycolytic flux and shut down the citric acid cycle, even in the
presence of oxygen ( 22 , 24 ) . This is reflected by an increased
expression of glycolytic enzymes in solid cancers ( 26 ) and in the
presence of abnormally high levels of lactic acid ( 27 ) , the final
product of anaerobic glycolysis. Anaerobic glycolysis in the pres-
ence of oxygen, also known as the Warburg effect, seems to be
promoted by the activation of signalling pathways, which in nor-
mal cells mediate the biological functions of growth factors and
insulin; the PI3K/Akt pathway seems to have a prominent role
in this respect ( 22 ) . However, not all cancers may deregulate this
pathway ( 28 ) . Thus in cancers with wild-type PI3K/Akt, muta-
tions on the Myc oncogene promote the utilization of glutamate
as the preferred energy source instead of glucose ( 24 ) . These find-
ings were possible because of the use of an LC-MS-based strategy
to quantify the fluxes of glycolytic and glutamate catabolic path-
ways ( 24 ) .
The connection between oncogenic signalling and metabolic
pathways is also exemplified by a LC-MS-based study ( 23 ) , which
showed that a single mutation on the oncogene K-Ras at codon
13 produced markedly different metabolic phenotypes than did
mutations on K-Ras at codon 12. Thus the mutation on codon
13 made cells increase the pentose phosphate pathway flux,
while mutations on codon 12 diverted metabolism to glycolysis
( 23 ) . Taken together, these studies illustrate the heterogeneous
nature of metabolic alterations in cancer cells and the connection
between metabolism and signalling. Metabolomics studies based
on LC-MS are contributing to the understanding of this complex-
ity, but much more work is required to decipher how the different